Re: Foreign join pushdown vs EvalPlanQual - Mailing list pgsql-hackers

>>> On Mon, Oct 19, 2015 at 3:45 AM, Etsuro Fujita
>>> <fujita.etsuro@lab.ntt.co.jp> wrote:
>>>> As Tom mentioned, just recomputing the original join tuple is not good
>>>> enough.  We would need to rejoin the test tuples for the baserels even if
>>>> ROW_MARK_COPY is in use.  Consider:
>>>>
>>>> A=# BEGIN;
>>>> A=# UPDATE t SET a = a + 1 WHERE b = 1;
>>>> B=# SELECT * from t, ft1, ft2
>>>>        WHERE t.a = ft1.a AND t.b = ft2.b AND ft1.c = ft2.c FOR UPDATE;
>>>> A=# COMMIT;
>>>>
>>>> where the plan for the SELECT FOR UPDATE is
>>>>
>>>> LockRows
>>>> -> Nested Loop
>>>>      -> Seq Scan on t
>>>>      -> Foreign Scan on <ft1, ft2>
>>>>           Remote SQL: SELECT * FROM ft1 JOIN ft2 WHERE ft1.c = ft2.c AND ft1.a
>>>> = $1 AND ft2.b = $2
>>>>
>>>> If an EPQ recheck is invoked by the A's UPDATE, just recomputing the
>>>> original join tuple from the whole-row image that you proposed would output
>>>> an incorrect result in the EQP recheck since the value a in the updated
>>>> version of a to-be-joined tuple in t would no longer match the value ft1.a
>>>> extracted from the whole-row image if the A's UPDATE has committed
>>>> successfully.  So I think we would need to rejoin the tuples populated from
>>>> the whole-row images for the baserels ft1 and ft2, by executing the
>>>> secondary plan with the new parameter values for a and b.

Robert Haas wrote:
>>> No.  You just need to populate fdw_recheck_quals correctly, same as
>>> for the scan case.

I wrote:
>> Yeah, I think we can probably do that for the case where a pushed-down
>> join clause is an inner-join one, but I'm not sure that we can do that
>> for the case where that clause is an outer-join one.  Maybe I'm missing
>> something, though.

On 2015/10/20 15:42, Kouhei Kaigai wrote:
> Please check my message yesterday. The non-nullable side of outer-join is
> always visible regardless of the join-clause pushed down, as long as it
> satisfies the scan-quals pushed-down.

Sorry, my explanation was not correct.  (Needed to take in caffeine.) 
What I'm concerned about is the following:

SELECT * FROM localtab JOIN (ft1 LEFT JOIN ft2 ON ft1.x = ft2.x) ON 
localtab.id = ft1.id FOR UPDATE OF ft1

LockRows
-> Nested Loop     Join Filter: (localtab.id = ft1.id)     -> Seq Scan on localtab     -> Foreign Scan on <ft1, ft2>
     Remote SQL: SELECT * FROM ft1 LEFT JOIN ft2 WHERE ft1.x = 
 
ft2.x FOR UPDATE OF ft1

Assume that ft1 performs late row locking.  If an EPQ recheck was 
invoked due to a concurrent transaction on the remote server that 
changed only the value x of the ft1 tuple previously retrieved, then we 
would have to generate a fake ft1/ft2-join tuple with nulls for ft2. 
(Assume that the ft2 tuple previously retrieved was not a null tuple.) 
However, I'm not sure how we can do that in ForeignRecheck; we can't 
know for example, which one is outer and which one is inner, without an 
alternative local join execution plan.  Maybe I'm missing something, though.

Best regards,
Etsuro Fujita